Convertible Lenses
At one time, many large format lenses were convertibles. These lenses were attractive because they are, essentially, two or more lenses in one. Removing the front element of the lens increases the focal length. With a triple convertible, rearranging the elements according to the manufacturer's design gives the photographer three different focal lengths.
Without going into great detail regarding these interesting lenses, it is safe to say that there are compromises with these designs. While the shorter focal length is usually a good lens, "converting" it generally is considered to reduce image quality. One way to improve image quality when using black and white film with a "converted" lens is to use a yellow filter.
Many photographers feel that these older convertible lenses are greatly inferior to today's excellent range of single focal length view camera lenses. On the other hand, it is undeniable that many outstanding photographs were made with convertibles. Some of Ansel Adams' classic photographs, including "Moonrise," Hernandez New Mexico, "Clearing Winter Storm," Yosemite National Park, "Aspens," Northern New Mexico, and "Mount Williamson" from Manzanar, California were all taken with a Cooke Series XV triple convertible lens. (We should all be similarly hampered by such inadequate equipment!)
Users can often find excellent values in used convertible lenses, which will give them a fine prime focal length, and, at the very least, a serviceable longer focal length when converted. If you are interested in a used convertible, older Schneider Symmar lenses (not the APO-Symmar or the Symmar-S models) are good choices. Other classic convertible lenses include the Bausch and Lomb Protar, and the Turner Reich triple convertible.
Wisner Classic Manufacturing offers the only newly manufactured convertible lenses available today, called the Wisner Convertible Plasmat. The set is a replica of the Protar, and is made up of several single lens cells that can be combined to create a variety of focal lengths. The 4x5 and 5x7 sets are mounted on a Copal No. 1 shutter. The 8x10 set is mounted on a Copal No. 3 shutter. A yellow filter is required with black and white film. An item called a "zero power corrector" is required to correct color fringing and other issues when using color film. The sets and accessories are quite expensive. Schneider manufactures the optics to Wisner's specifications. The lenses are assembled and tested by Wisner.